Impact of thalamic deep brain Stimulation on lexical processing

The contribution of subcortical structures to language-related processing is controversial, but interestingly Deep Brain Stimulation for the treatment of Movement Disorders can go along with reduced lexical capacities. Against this background, word processing functions were assessed as a function of thalamic versus subthalamic Deep Brain Stimulation. Both on and off stimulation, 14 (2/12 f/m; 64.71 (± 7.88) years) Parkinson's disease patients with subthalamic and 10 (6/4 f/m; 68.00 (± 10.39) years) Essential Tremor patients with thalamic Deep Brain Stimulation engaged in a Lexical Decision Task, requiring the differentiation of acoustically presented words from non-words. These stimuli were preceded by non-target words, either phonemically or semantically related or unrelated to the following presentation. During task performance, event-related potentials were recorded. This allowed for the determination of (i) lexical decision time, (ii) priming effects, and (iii) the N400 as neurophysiological correlate of lexical stimulus processing. Measurements were additionally performed in 19 (9/10 f/m; 29.95 (± 5.60) years) young and 12 (5/7 f/m; 66.00 (± 7.30) age matched healthy subjects. Overall, the performance speed in the Lexical Decision Task correlated with N400 peak latency (rs = 0.49; p = 0.003). N400 amplitudes were abnormally low in the group of patients with Parkinson's disease. With respect to on-off effects, active compared to inactive stimulation prolonged word decisions (p = 0.010) and reduced N400 potentials in thalamic (p = 0.018), but not subthalamic Deep Brain Stimulation (see Fig. 1). This was irrespective of word priming. In conclusion the expression of N400 and Lexical Decision Task performance are influenced by thalamic state functions, modulated by Deep Brain Stimulation. This is compatible with the idea of thalamo-cortical engagement of word-related processing, as suggested by 6 influential biolinguistic models. In clinical terms, the findings specify cognitive sequels of Deep Brain Stimulation in a target-specific way.